This invention relates in general to gas sensors and in particular to a device for increasing the selectivity of gas sensors that employ field effect transistors (“FETs”).
FIGS. 3 and 4 herein and German Patents DE 4239319, DE 19956744, and DE 19956806 disclose FET gas sensors of hybrid construction, which read the work function of a gas-sensitive material using a field effect transistor. These sensors have a number of different applications. On the one hand, they can be operated at ambient temperature, or at slightly elevated temperatures, and they therefore permit low-power operation with batteries or direct connection to data bus lines without the use of auxiliary power. On the other hand, a large number of different materials can be used as detection materials with this sensor structure, so that a previously unachieved bandwidth of gases can be detected with these sensors. Economical manufacture is possible with readily automated techniques because of their simple structure. Since the control electronics can be integrated into the Si chip with little added cost, the costs of gas sensor systems having the control electronics are lower than for other sensor technologies.
These gas sensors are also subject to the problems of cross sensitivities; in other words, other gases that exist in the application can cause interference with the sensor signal. That is, the sensor reacts to interfering gases that may distort a concentration of the measured gas (i.e., direct cross sensitivity). Similarly, sensitivity to the target gas can be modified by the presence of an interfering gas (i.e., indirect cross sensitivity). Both effects lead to distortion of the desired sensed values and can impair or even prevent usability in an application, depending on the requirement profile.
A first approach to eliminating existing drawbacks is an intelligent signal processor on the system level. In this case, the attempt is made to eliminate the consequences of incorrect measurements by a plausibility consideration for the sensor signal, for example. This approach is not possible with indirect cross sensitivity.
A second approach uses an additional sensor element that is sensitive to the target gas and corrects the indicated value of the primary sensor element with this auxiliary information from the additional sensor element during the test. This is an approach that was actually pursued with array-capable sensors (multiple system) like GasFETs; see for example German Patent DE 19956806. This variant is always associated with distinctly greater effort and higher cost. The consequences of indirect cross sensitivity, however, cannot be eliminated with this approach, or only with great effort, for example with large sensor arrays or extensive calibration models.
A third approach comprises further developing and optimizing the sensor material so that selective detection of the target gas is achieved. This can be achieved for some specific applications, for example see German Patents DE 19926747, DE 19849932, and DE 19708770. However, it cannot be assumed from this that this is possible for most detection tasks.
A fourth approach of the prior art is related directly to the FET gas sensors, for example see German Patent DE 19849932. It utilizes the geometry of this structure to produce very high electric field strengths on the surface of the sensor layer by applying manageable voltages, for example 10V, to the suspended gate electrode because of the small air gap. These affect the adsorption properties of the detected molecules. The influence of an interfering gas can be eliminated by comparing the measured signals with various electric field strengths on the surface of the sensor layer. This approach is not universally applicable, such that its utility is limited to a few special cases.
A fifth prior art approach constitutes a self-explanatory procedure for eliminating cross sensitivities. The use of filters that are mounted between the gas mixture to be detected and the gas sensor is proposed for this purpose. They are permeable to the target gas but do not allow gases that cause cross sensitivities to reach the sensor. Exemplary embodiments in this case are catalytic filters as disclosed in German Patent DE 19926747, with the concentrations of interfering gas being actively removed by a chemical reaction. Gas sensors based on heated, semiconducting metal oxides are often combined with an activated charcoal filter. This removes gases that cause cross sensitivities by adsorbing them on the large internal surface area of the filter material, but with the target gas being allowed to pass through the filter and be detected by the sensor. A widely used example of such sensors is gas sensors for detecting toxic gases, CO for example, or explosive gases, for example escaping natural gas/CH4 in domestic atmospheres. Alcohol vapors occurring in the household often interfere with their measurement signals as disclosed in German Patent DE 19926747. Filters are also used frequently for electrochemical gas sensors. Activated charcoal is a very good absorber for alcohol vapors, for example, in long-term operation in these applications. However, the filter can become saturated, so that the filter loses its activity and the interfering gas ultimately does reach the sensor and is detected.
There is a need for a FET-based gas sensor that prevents distortion of the measured signal by cross sensitivities.